Offset and gain errors introduced during conversions of an analog signal to a digital signal by an ADC are corrected for by calibration of the ADC. On multi-channel converters which share a single ADC circuit, a single calibration coefficient storing circuit may be provided for storing calibration codes for calibrating the ADC circuit only to correct for offset and gain errors introduced by the ADC circuit. Alternatively, a calibration coefficient storing circuit may be provided for storing respective calibration codes for simultaneously correcting for errors introduced by the ADC circuit and corresponding ones of the respective input channels. In the latter case the calibration code for each channel comprises an offset correction code for correcting for offset errors introduced by both the ADC circuit and the corresponding input channel, and a gain correction code for correcting for gain errors introduced by both the ADC circuit and the corresponding channel. Such ADCs suffer from a number of disadvantages. Where only a single calibration coefficient storing circuit is provided for storing an offset and gain calibration code for correcting for the ADC circuit only, no correction can be made in the ADC for offset and gain errors introduced by the respective input channels. Where the calibration coefficient storing circuit is provided for storing a plurality of offset and gain calibration codes for correcting for offset and gain errors introduced by the combination of the ADC circuit and the respective corresponding input channels, while both ADC circuit and channel introduced offset and gain errors may be corrected for, they must be corrected for simultaneously. This presents a problem, in that unless the input channels are on-chip channels, factory calibration is of little use, since if calibration for off-chip channels is to be carried out, this will have to be carried out on-site, and any factory calibration codes relating to the ADC circuit and the channels will have to be overwritten when calibration for off-chip channels is being carried out. This is undesirable, since the integrity of an accurate factory calibration can be lost as a result of stored accurate factory calibration codes being overwritten when off-chip channel calibration is being carried out by a user. Additionally, such calibration coefficient storing circuits do not permit manual entry of calibration codes without loss of the integrity of an accurate factory calibration.
There is therefore a need for a multi-channel ADC which overcomes these problems. There is also a need for a method for calibrating a multi-channel ADC which also overcomes these problems. There is also a need for an ADC, multi-channel or single channel and a method for correcting digital output signals for errors introduced by an ADC circuit of the ADC and/or errors introduced by one or more of the analog input channels to the ADC.
The present invention is directed towards providing such a multi-channel ADC and a method for calibrating the ADC. The invention is also directed towards providing an ADC, multi-channel or single channel which facilitates correcting for errors in the digital output signal introduced by an ADC circuit in the ADC and/or one or more of the analog input channels. The invention is also directed towards providing a method for correcting for such errors in digital output signals of an ADC.